Diversity of spatial relationships of the claustrum and insula in branches of the mammalian radiation

Using digital images of stained sections, we conducted a comparative survey of the visible gross morphology of the insular cortex and the claustrum in 26 mammalian species, representing most of the major mammalian radiations. We observed several features that are conserved in the mammals that we examined, including the absence of expansion of the insular cortex into out‐branching lobes, the adherence of the claustrum to the fundus of the rhinal sulcus and to endopiriform cellular regions, and several features that varied, including the colocation of the insular and claustrocortex, the cortical origin of the operculations that cover the insula, the locations of large accumulations of claustral cells, and the presence of thin extensions of groups of claustral cells. We suggest that the final shape of the claustrum is highly influenced by the expansion of two medially adjacent subcortical structures, the striatum and amygdala. We further suggest that the neighboring locations of the claustrum and insula may be due to a dependence of each of these structures upon proximity to olfactory cortex, rather than to any connectional relationships between them.